With rapid development of various types of electronic devices, there is a tendency that these devices are increasingly produced in compact and lightweight design. In particular, the tendency toward compact and lightweight design is more remarkable in the portable type electronic devices to be used in camera-integrated video tape recorder, portable telephone set, note-book personal computer, palm-top computer, etc. The component parts for these devices are also increasingly produced in compact and lightweight design. The means for mounting the electronic components are also changing to surface mounting technology (SMT). Small components such as capacitor and resistor are called “chip components”.
Multilayer Ceramic Capacitor (MLCC) is a widely used typical chip component.
The Electronic Industry Association (EIA) prescribes a standard for temperature coefficient of capacitance (TCC) for a type of MLCC known as the X7R Capacitor. The X7R characteristic requires that with reference to the capacitance at 25° C., the change of the capacitance (i.e. TCC) be within ±15% over the temperature range from −55° C. to 125° C.
When MLCC based on BaTiO3 were sintered in air at high temperatures, it is required to use noble metals (e.g., palladium, platinum, etc.) as internal electrodes, which do not melt and do not oxidize even when being fired in an atmosphere with a high partial pressure of oxygen. However, the use of such noble metals becomes a barrier to cut down the production cost of multilayer ceramic capacitors. For example, the cost of internal electrodes occupies about 30 to 70% of the production cost of multilayer ceramic capacitors.
For the reasons mentioned above, it is preferred to use a base metal such as Ni, Fe, Co, W or Mo as the material of internal electrodes. However, if such base metals are used as a material for internal electrodes and fired in the conventional firing conditions of the dielectric ceramic materials, they oxidize easily and loose functions as the internal electrodes. Thus, in order to use such a base metal as a material for internal electrodes of multilayer ceramic capacitors, it is required to use a dielectric ceramic material which is never semiconductorized even if being fired in a neutral or reducing atmosphere with a low partial pressure of oxygen, and which has a sufficient insulation resistance and good dielectric properties.
Usually, the dielectric sheets and the internal electrodes were stacked together. The laminated bodies were then sintered at 1200° C.–1400° C. under reducing atmosphere created by N2—H2—H2O gas system and then annealed at 1000° C.–1100° C. under weak-oxidation atmosphere with a partial pressure of oxygen of 10−6 atm or higher.
To meet the requirements of X7R, there have been proposed some non-reducible dielectric ceramic materials. For example, JP-A-63-103861 discloses a composition of BaTiO3—MnO—MgO-Rareearth element system. However, this dielectric ceramic composition is of no practical use since its insulation resistance and temperature coefficient of capacitor are affected by the grain size of the main component BaTiO3, thus making it difficult to control the composition to obtain stable dielectric properties.
In U.S. Pat. No. 5,403,797A, a composition of BaTiO3—Y2O3—MgO—V2O5 is disclosed. Although its dielectric constant at room temperature is greater than 2500, its sintering temperature is higher than 1350° C. and the TCC at −55° C. is near −15% with dielectric loss higher than 2%. So this composition is not suitable for industrial production.